Railway car trucks such as conventional three-piece freight car trucks have conventionally used sliding friction to dissipate energy during relative movement between the truck bolster and the side frame. Specifically, spring loaded friction shoes or wedges have been disposed between the bolster and the side frame so as to bear on generally vertical side frame column surfaces, and to slide on the column surfaces in response to relative movement of the bolster, both vertical and horizontal, with respect to the side frames. The resultant sliding friction between the friction shoes and the side frame column surfaces dissipates some of the energy associated with the movements of the bolster and side frames, and the car body. Such energy dissipation can improve ride quality and control truck hunting and truck warping.
A friction shoe typically is formed as a wedge which is biased into a bolster pocket against upwardly converging bolster pocket and side frame column surfaces, as is well known. To maintain such friction wedges in engagement with the bolster pocket and side frame column surfaces, mechanical spring force of either constant or varying magnitude has been used. One common means of friction wedge restraint has been an elongated compression spring extending between the side frame window floor and an undersurface of the friction wedge such that the biasing force which retains the friction wedge varies with the relative vertical position of the bolster with respect to the side frame.
The art is replete with examples of railway truck friction wedge arrangements, and although conventional friction wedges have generally been suitable for their intended purpose, practitioners in the art have continually sought improvements in friction wedge performance. For example, friction wedges can degrade vertical ride quality because they respond to any relative movement of the bolster with respect to the side frame with the same frictional restraint. That is, all relative bolster to side frame displacements of any velocity, and whether large in magnitude or small, result in full frictional damping force per unit of frictional sliding movement by the friction wedges on the column surfaces. This can degrade ride quality; however, if the friction forces evolved by conventional friction wedges are kept small out of concern for ride quality, the result may be insufficient damping for the more violent bolster movements with respect to the side frame. Such movements may require greater energy dissipation in order to prevent such undesirable results as spring bottoming or evolution of harmonic responses resulting from periodic force inputs.
A great many friction shoe structures and railway trucks adapted to use them are known from the prior art, including U.S. Pat. Nos. 4,109,586, 2,352,693 and 2,737,905.
To deal with the described limitations of frictional damping, one approach has been to add supplemental damping, for example viscous damping such as provided by hydraulic dampers, to further restrain relative bolster to side frame movement in railway trucks. The damping response of a hydraulic damper generally is velocity dependent so that for small and/or slow relative movements, the hydraulic damper develops limited restraint whereas, in response to larger velocity, amplitude or frequency of relative movement, the hydraulic damper evolves greater restraining force. The prior art contains numerous examples of hydraulic dampers applied in railway trucks including U.S. Pat. Nos. 4,936,226, 4,198,911, 3,773,147, 4,132,176, 2,573,165, 2,284,696, 1,983,088, and the above-referenced U.S. Pat. No. 4,109,586.
Like conventional friction wedges, conventional hydraulic railway truck dampers also have been generally suitable for their intended purposes although improvements have nonetheless been continually sought. For example, the velocity dependent damping of conventional hydraulic dampers typically has been single-acting, which means that for a given level of energy removal twice the force is necessary as that which would be necessary if the hydraulic damper were double-acting. In addition, prior hydraulic dampers have acted directly between a railway truck bolster and side frame, thus being completely independent of friction wedge operation. Although often helpful in controlling evolution of harmonic responses, some known railway truck hydraulic dampers are less effective in improving vertical ride quality. Furthermore, frequent hydraulic damper operation in response to rapid, but small amplitude relative bolster to side frame movements during normal running may result in considerable heat build up in the damper with consequent reduction in damper service life.